1. Field of the Invention
The invention relates to a light source device using a discharge lamp with high radiance (HID lamp), such as a high pressure mercury discharge lamp, a metal halide lamp or the like, which is used, for example, as a light source for a projector.
2. Description of the Prior Art
In a light source device for an optical device, such as a liquid crystal projector, a DLP® projector (Texas Instruments) or the like, a discharge lamp, such as a HID lamp or the like, is used. With respect to driving the discharge lamp, direct current and alternating current drives exist. In the direct current drive, the polarity of the electrodes of the two poles of the discharge lamp, i.e., the cathode and the anode, is not changed. In the alternating current drive, the relation between the anode and cathode is changed essentially periodically. In the alternating current drive, with reference to the rate of the polarity change, i.e., with reference to the control frequency, driving with a wide frequency from a few dozen hertz to a few megahertz is possible.
In one such discharge lamp, the discharge space acts as an acoustic resonance cavity. In the case in which the frequency component of this resonant frequency and the frequency component of the feed current agree with one another or approach one another, a phenomenon which is called “acoustic resonance” occurs.
Generally, there are several resonant frequencies, corresponding to the lamp shapes. The value of the resonant frequency in itself depends on the dimensions and on the acoustic velocity of the gas within the discharge space. Therefore, the resonant frequency changes for each type of lamp. It also changes for each individual lamp of the same lamp type when the shape dimensions of the discharge space have variances. Furthermore, the resonant frequency also changes in the same lamps when the lamp temperature changes.
When a feed device is driven by the above described direct current drive, resonance occurs with respect to current ripple and its harmonic component. When driving takes place by the above described alternating current drive, resonance occurs with respect to the control frequency and its harmonic component. Furthermore, if there is ripple in the drive half-period, resonance occurs also with respect to its ripple component and its harmonic component.
When acoustic resonance occurs, the discharge arc within the discharge space oscillates. Therefore, there are cases in which the illuminance of the projector image has flicker, or in which, in extreme cases, the discharge disappears. As was described above, the frequency of the acoustic resonance changes differently according to lamp conditions. Furthermore, with respect to the different frequency components of the feed device, resonance occurs. Therefore, it was very difficult to separate the frequency of the acoustic resonance of the lamp and the frequency components of the feed device stably from one another.
In the prior art, such as, for example, in Japanese patent disclosure document SHO 63-58793, it is disclosed that the frequency of the ripple component and the degree of ripple are limited. However, as was described above, it is difficult to stably separate the frequency of the acoustic resonance of the lamp and the frequency components of the feed device from one another. Therefore, there were also cases in which implementation was not possible. Furthermore, for example, in Japanese patent disclosure SHO 60-262392, it is disclosed that the frequency band in which no acoustic resonance at all occurs, and the frequency band in which acoustic resonance can occur, are switched with a given period. This, however, cannot be used for the case in which the frequency band in which no acoustic resonance at all forms is not present.
In Japanese patent publication 2001-505360 and in U.S. Pat. No. 5,859,505, it is disclosed that the stability of lamp operation is measured and the frequency is dynamically switched. However, here, the disadvantages of a complicated arrangement and high costs arose.
Furthermore, in this frequency switching system, there were the disadvantages that, before and after switching of the frequency, the characteristics change, and that, in this way, there is the possibility of changing the emission amount when there is a peripheral circuit with frequency characteristics, such as a noise filter, a partial resonance circuit, an artificial resonance circuit or the like. To prevent this change from occurring, another concept is needed, for example, an increase in the speed of the feedback response of the lamp wattage control or a similar concept. This caused the disadvantages of complex circuitry and increased cost.